In general, a lithographic printing plate consists of a lipophilic image part which receives an ink in the course of printing and a hydrophilic non-image part (an ink-unreceiving part) which receives a fountain solution. In lithographic printing, printing is made by taking advantage of the properties of water and an oily ink repealing each other. As a result, the ink unevenly adheres to the surface of the lithographic printing plate, i.e., the ink adheres exclusively to the mage part. Next, the ink is transferred onto a print substrate such as paper, thereby completing printing.
To construct a lithographic printing plate, it has been a common practice to use a lithographic printing plate precursor (a PS plate) having a lipophilic photosensitive resin layer (an image recording layer) formed on a hydrophilic support. A lithographic printing plate is usually made by exposing a lithographic printing plate precursor via a master such as a lith film and dissolving and removing the image recording layer in the non-image part with the use of an alkaline developing solution or an organic solvent while remaining the image recording layer in the image part to thereby expose the surface of the hydrophilic support.
In the plate making process with the use of a conventional lithographic printing plate precursor, it is necessary to employ the step of dissolving and removing the non-image part with a developing solution after the exposure. Recently, it is a technical problem to omit or simplify such an additional wet treatment as the development step. In recent years, the disposal of waste solutions discharged in association with the wet treatment has attracted much industrial attention from the viewpoint of global environment. That is to say, there is an increasing demand for omitting the wet treatment from the environmental viewpoint.
As one of simplified plate making methods, there has been proposed a so-called onboard development method, which comprises, after exposing, removing a non-image part on a printing machine to give a lithographic printing plate. In this method, use is made of an image recording layer allowing the removal of a non-image part of a lithographic printing plate precursor during the common printing process
Specific examples of the onboard development method include a method with the use of a lithographic printing plate precursor having an image recording layer which can be dissolved or dispersed in a fountain solution, an ink solvent or am emulsion of a fountain solution with an ink; a method of mechanically removing an image recording layer by contacting it with rollers or a blanket cylinder of a printing machine; and a method wherein the cohesive force of an image recording layer or the adhesion between the image recording layer and a support is weakened by the penetration of a fountain solution, an ink solvent or the like and then the image recording layer is mechanical removed by contacting it with rollers or a blanket cylinder.
Unless otherwise noted, the term “development treatment” as used herein means the step of removing a non-image part of a lithographic printing plate precursor by using an apparatus (usually an automatic developing machine) other than a printing machine and contacting the printing plate precursor with a liquid (usually an alkaline developing solution) to thereby expose the surface of a hydrophilic support. On the other hand, the term “onboard development” means the method and step of removing a non-image part of a lithographic printing plate precursor by using a printing machine and contacting the printing plate precursor with a liquid (usually a printing ink and/or a fountain solution) to thereby expose the surface of a hydrophilic support.
In the case of using an image recording layer of the conventional image recording system with the use of ultraviolet light or visible light, however, the image recording layer would not sufficiently fixed after the exposure. Therefore, it is necessary to employ some troublesome procedures, for example, storing a printing plate precursor in a completely light-blocked state or at a constant temperature.
With the recent spread of digitalization techniques of electronically processing, accumulating and outputting image data with a computer, various image output systems appropriate for these digitalization techniques have been put into practical use. Thus, there have been developed computer to plate techniques whereby a printing plate is directly exposed, without resort to a lith film, by scanning highly directional radiant rays such as laser beams on the printing plate in accordance with digitalized image data. Accordingly, it is one of important technical problems to obtain a lithographic printing plate appropriate for these systems.
As discussed above, it has been more urgently required in recent years to simplify the plate making procedures, employ a dry system and omit the wet treatment from the viewpoints of global environment and adaptation to digitalization.
In recent years, high output lasers such as semiconductor lasers and YAG lasers are available less expensively. Accordingly, it is highly expected that these high output lasers are useful as image recording light sources in a method of producing a lithographic printing plate precursor by scanning exposure which can be easily incorporated into the digitalization technology.
In the conventional plate making method, a photosensitive lithographic printing plate precursor is imagewise-exposed at a low to medium illumination intensity and thus an image is recorded via an imagewise change in a physical property due to a photochemical reaction in the image recording layer. In the method with the use of the high-output lasers as described above, in contrast thereto, an area to be exposed is radiated with much light energy within an extremely short time. Thus, the light energy is efficiently converted into heat energy, thereby inducing a heat change such as a chemical change, a phase change or a morphological or structural change in the image recording layer. Using this change, an image is recorded. That is to say, the image data is input by the light energy such as laser beams, while image recording is carried out by using not only the light energy but also the reaction by heat energy. Such a recording system with the use of the heat generated by high power density exposure is called “heat mode recording” and the conversion of light energy into heat energy is called “photothermal conversion”.
Large merits of the plate making method with the use of the heat mode recording reside in that the image recording layer would not become light-struck at an ordinary illumination level such as room light, and that fixation of an image recorded by high illumination exposure is not essentially required. That is, a lithographic printing plate precursor to be used in heat mode recording is free from any fear of light-struck due to room light before the exposure and the fixation of an image is not essentially required after the exposure. When a plate making process, wherein an image recording layer insolubilized or solubilized by the exposure using high output lasers is employed and the imagewise-exposed image recording layer is employed as a lithographic printing plate, is carried out by the onboard development method, for example, it is expected that a printing system in which an image is not affected even though it is exposed to environmental room light after the exposure could be obtained. Accordingly, it is expected that use of the heat mode recording enables the acquisition of a lithographic printing plate precursor appropriately usable in the onboard development method.
Thanks to the-remarkable progress in laser technology in these days, high-power and small sized semiconductor lasers and solid lasers emitting infrared rays of 760 nm to 1200 nm in wavelength are easily available. These infrared lasers are highly useful as recording light sources in direct plate-making of digital data from computers, etc.
However, many of photosensitive recording materials useful as an image recording layer in practice have photosensitive wavelength within the visible light region of 760 nm or below and, therefore, image recording cannot made with infrared laser beams. Therefore, it has been required to develop a material allowing infrared laser image recording.
To satisfy such a requirement, a lithographic printing plate precursor having an image recording layer, in which hydrophobic thermoplastic polymer particles are dispersed in a hydrophilic binder, provided on a hydrophilic support is proposed (see, for example, Japanese Patent No. 2938397). This lithographic printing plate precursor is exposed by infrared laser beams and thus the hydrophobic thermoplastic polymer particles are thermally bonded together to form an image. Then it is loaded on a cylinder of a printing machine and a fountain solution and/or an ink are supplied to it. Thus, onboard development can be carried out.
Although this system wherein an image is formed by bonding fine particles merely via heat fusion shows favorable onboard development properties, the obtained image is very poor in strength (adhesion to the support) and thus no sufficient printing tolerance can be established.
There is also proposed a lithographic printing plate precursor which contains microcapsules having a polymerizable compound encapsulated therein on a hydrophilic support (see, for example, JP-A-2001-277740 and JP-A-2001-277742).
Moreover, a lithographic printing plate precursor which has a photosensitive layer containing an infrared absorbing agent, a radical polymerization initiator and a polymerizable compound on a support is proposed (see, for example, JP-A-2002-287334).
The above-described methods using polymerization are characterized in that relatively high image strength can be achieved thereby, since chemical bond density in an image part is higher than that of an image formed by the heat fusion of fine polymer particles. From a practical viewpoint, however, the onboard development properties, printing tolerance and polymerization efficiency (sensitivity) still remain insufficient. Therefore, these methods have not been put into practical use yet.
There has been proposed to add a phosphoric ester monomer to a lithographic printing plate precursor of the photopolymerization initiator system (see, for example, JP-A-11-30858). The phosphoric ester moiety of the phosphoric ester monomer has a function of adsorbing to the support surface. The monomer moiety of the phosphoric ester monomer is hardened by photopolymerization. By adding the phosphoric ester monomer to the photosensitive layer, therefore, the adhesion of a hardened image to the support is enhanced in an exposed part and, in its turn, the printing tolerance is improved.